This is a must-do in Flagstaff. Although the admission ticket is a bit pricey, it is good for the entire day, so take a tour with the expert guides during the day to see everything, then return at night when they have the telescopes out and trained on specific stars and constellations. We had a great tour guide with a good sense of humour who explained to us about the history of Lowell, the discovery of Pluto (ah, poor Pluto--still a touchy subject at the Observatory), and showed us the different telecopes around the hilltop campus. It's a good tour of about 45-60 minutes, and you are free to wander the campus after, although you can only get into some of the telecope buildings with the tour. At night they have the telescopes out, most on the open deck observatory, with staff eager to explain and show you the stars. We were also lucky that the night we went, the original Clark telecope (which Lowell used to discover Pluto) was operational and we got to peer through that. So cool, even for non-astronomy buffs. Note that the observatories are not heated, so plan accordingly if you visit in the winter.